Recycling of Low Grade Clothing Waste - Oakdene Hollins

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© Oakdene Hollins Ltd, Salvation Army Trading Company Ltd Nonwovens Innovation & Research Institute Ltd September 2006 Motion of gas molecules in aerogel Molecular collisions give rise to heat transfer in a still gas. Faster (hot) moving molecules collide with slower (cold) moving molecules and pass on some of their heat energy. This collision process can be prevented by a barrier so that the molecules will not collide with each other, but rather will collide with the barrier from which they rebound and therefore retain their heat energy. The average distance which a gas molecule travels before it collides with another molecule is defined as the mean free path. A molecule moving through a gas collides with other molecules in a random fashion. To prevent these collisions, the barriers need to be separated by a distance smaller than the mean free path. The mean free path of gas molecules l m0 2 g l m 0 in free space obeys the equation: 1 = [1] 2n πd g Where: ng dg = Number density of gas molecules = Diameter of gas molecules The mean free path of gas molecules in aerogels lm can be expressed as: 1 lm = [2] 2 2n πd + S ρ / Π g g s por Where: Ss = Specific surface area of the aerogel defined as surface area per unit mass ρ por = aerogel density П = Porosity defined as the volume of the pores divided by the total volume of the matrix As may be observed, the mean free path of gas molecules in an aerogel is smaller than in free space because of the addition of Ssρ por / Π to the denominator of equation [1]. As a result of the retarded motion of the gas molecules in the aerogel, the thermal conductivity of the gas in an aerogel is about half that of the gas in free space. For Defra Page 134
© Oakdene Hollins Ltd, Salvation Army Trading Company Ltd Nonwovens Innovation & Research Institute Ltd September 2006 The process parameters for the aerogel composite are given below: Recovered Fibre 66.7% Bicomponent fibre 30.0% Aerogel 3.3% Aerogel type Cabot OT01N Web weight 150gm ‐2 Oven temperature 150°C Oven belt speed 1 m/min Calendering YES Activated Carbon It is the extended surface area of activated carbons that imparts the versatile adsorbent properties of activated carbons. Commercially available activated carbons have specific surface areas in the order of 800‐1500m 2 /g. However studies on biomass flax fibre, chemically activated using zinc chloride, have produced activated carbons with surface areas up to 2400m 2 /g 89 . This large surface area is the result of the activation process which involves oxidising carbonaceous char in an atmosphere of air, CO2 or steam at elevated temperatures (800‐900°C). The major raw materials are wood, coal, lignite, coconut shell and peat. These materials come in granular and powdered form. The granular form is characterised by a large internal surface area and small pores. The powdered form generally exhibits larger pore diameters than the granular form at the expense of smaller internal surface area. Powdered ACs have diameters less than 100μm, typically 15‐25μm on average. Owing to their large external surface the adsorption rate is very high. Conversely, granulated ACs are associated with lower adsorption rates. The exact processing conditions are below: Recovered Fibre 60.7% Bicomponent fibre 33.3% Activated Carbon 6.7% Grade of AC Chemviron Pulsorb FE06104A Web weight 100gm ‐2 Oven temperature 150°C Oven belt speed 1 m/min Calendering YES For Defra Page 135
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Recycling of Low Grade Clothing Waste - Oakdene Hollins

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